This invention relates to plant disease resistance, in particular to plant genes conferring pathogen resistance.
Whether a plant is resistant or susceptible to attack by a given pathogen is frequently under the control of a single, dominant resistance gene (Flor, Annu. Rev. Phytopathol. 9:275-296, 1971). Resistance gene products are thought to recognize signal molecules produced by the pathogen and respond by initiating rapid changes in host cell physiology and metabolism that directly inhibit pathogen growth.
Classic genetic analyses of disease resistance in plants have demonstrated that resistance to pathogens is often highly specific, requiring single corresponding genetic loci in both the plant and the pathogen. Several plant genes corresponding to these resistance loci have recently been cloned and characterized. The Pto gene from tomato, which confers resistance to Pseudomonas syringae pv. tomato, encodes a serine-threonine protein kinase (Martin et al., Science 262:1432-1436, 1993). The RPS2, RPM1 and RPP5 genes from Arabidopsis (Bent et al., Science 265:1856-1860, 1994; Mindrinos et al., Cell 78:1089-1099, 1994; Grant et al., Science 269:843-846, 1995), the N gene from tobacco (Whitham et al., Cell 78:1101-1115, 1994), and the L6 gene from flax (Lawrence et al., Plant Cell 7:1195-1206, 1995) all have several motifs suggestive of signaling roles for these proteins. These motifs include nucleotide binding sites (NBSs) and leucine-rich repeats (LRRs); thus, these proteins are commonly referred to as the NBS/LRR class. LRR motifs have been implicated in protein-protein interactions (Kobe and Deisenhofer, Trends Biochem. Sci. 19:415-421, 1994), raising the possibility that the LRR portion of these resistance gene products is required for specific recognition of a protein elicitor, or for interaction with other protein components of a signal transduction pathway. RPP5, L6 and N also contain N-terminal domains with homology to the mammalian interleukin 1 receptor (IL-1R) and the Drosophila Toll gene. These genes encode transmembrane receptor tyrosine kinases that act upstream of the rel family transcription factors NF-.kappa.B and Dorsal. A kinase with high levels of homology to Pto--the Pelle gene product--functions downstream of Toll in Drosophila.
It is possible that molecular recognition of an avirulent pathogen requires proteins with all of these motifs. The identification of Prf, an NBS/LRR protein required for function of the Pto gene, and of Xa-21, a gene that confers resistance to bacterial blight of rice and has both an LRR and a kinase domain, supports this hypothesis. This striking similarity between genes that confer resistance to bacterial, viral and fungal pathogens suggests that mechanisms of resistance are conserved within and among different plant species. Therefore, the signal transduction pathways utilized by the different resistance gene products may converge at some common step. However, genes that integrate different upstream molecular recognition signals and activate a hypothetical common downstream resistance pathway have not been previously identified.